The present invention relates to a traveling-wave type light amplifier which operates stably regardless of the polarization direction of an input light and has a gain characteristic of a wide band range. 2. Description of the Related Art
A light amplifier used in the field of optical communication now attracts more and more attention since it not only can directly amplify an input light, but can also simplify an optical communication system, reduce the cost thereof and improve the reliability thereof. In particular, a traveling-wave type semiconductor light amplifier draws much attention since it has a wide band gain and less dependency upon the wavelength of an incident light.
In a structure of a traveling-wave type light amplifier, it is important to lower reflectance at both ends of the waveguide region so as to suppress the ripples superposed on the output spectrum due to Fabry-Perot resonance. To lower such reflectance, three structures of traveling-wave type light amplifiers are known. A first has an anti-reflection film formed on the reflection surface, i.e., the cleaved end surface of the waveguide region; second has a window formed by cutting the waveguide region before the end portion of the substrate; and a third has both the anti-reflection layer and the window described above.
FIG. 1 is a sectional plan view showing a conventional traveling-wave type light amplifier which has a window structure. A substrate 1 is formed by stacking a plurality of semiconductor layers. A waveguide 2 is formed on the substrate 1, extending in the longitudinal direction of the substrate 1. As shown in FIG. 1, the waveguide 2 is cut near both end portions of the substrate 1 in the longitudinal direction. Window regions 3 are provided on the substrate 1 at both ends of the waveguide 2. Transition regions 2b are formed at both ends of a parallel region 2a having a uniform width to surpress the inner reflection of the traveling-wave from the boundary between the waveguide 2 and the window region 3. The transition regions 2b have been formed by etching the end portions of the waveguide 2 in taper shape (Published Unexamined Japanese Patent Application No. 59-165481). However, under this conventional method, it is impossible to control the thickness and the width of each end portion of the waveguide 2. If an anti-reflection layer (not shown) is formed on a cleaved surface in addition to the window region 3, the reflectance of the light amplifier can be further reduced. The cleaved surface is formed by cleaving the end face of the substrate 1 which opposes to the window region 3. In such a light amplifier, the resultant reflectance is in the order of 10.sup.-3 % (see OQ89-17, the Society of Electronic Communication).
In all cases, the width of the end portion of the waveguide 2 and the thickness thereof cannot be the same or of a desired value. The width is defined as the dimension which is perpendicular to the thickness direction and in parallel with the surface of the semiconductor substrate. Thus a cross section of the ends of the waveguide 2 is asymmetrical when the light amplifier is rotated by an angle of 90.degree.. Consequently, two linear polarized wave components extending at right angles to each other, i.e., a transverse electric (TE) wave and a transverse magnetic (TM) wave, are asymmetrical. Since the light outputted from the optical fiber is, in general, used as the incident light to the light amplifier, the polarization surface of the incident light is fluctuated. Hence, the total gain of the light amplifier changes due to the difference between the gain for the TE wave and the gain for the TM wave, as a result of the asymmetry. This gain difference generally ranges from 5 to 7 dB, unless the waveguide is tapered at both ends. Even if the waveguide is tapered at both ends, the difference is as much as 1 dB. Various methods have been proposed to suppress such influence. In one method, a polarization-preserving fiber is used to stabilize the plane of polarization. In another method, two or more light amplifiers are arranged in series or parallel such that the incident lights are amplified while the two linearly polarized wave components extend at right angles to each other. In either method, the optical system design is complicated and it is cumbersome to adjust the optical axes of the various components.
In order to reduce the difference, it has also been proposed that the waveguide can be made to have a width and a thickness which are of the same valve. This method is not recommendable, however, for the following reason. Generally, the waveguide of the light amplifier has a width of about 1 .mu.m and a thickness of about 0.1 to 0.2 .mu.m. If the waveguide is made as thick as it is wide, optical confinement is reduced, and its threshold current value is increased. The optical output of the light amplifier will then inevitably decrease and the light amplifier will not achieve a sufficient optical coupling. On the other hand, if the waveguide is made as wide as it is thick, its current density increases so greatly that the light amplifier is not reliable. In practice, it cannot be recommended that the waveguide be made merely to have a width and a thickness which are the same.
In addition, if the width and the thickness of the waveguide at the output end of the light amplifier are of different values, both the near-field pattern of the light-emitting point and the far-field pattern of the output beam will be asymmetrical. Inevitably, the coupling efficiency between the light amplifier and the optical fiber is reduced.